The present invention relates to an apparatus for indirectly measuring the concentration of UF.sub.6 in H.sub.2 by means of measuring the thermal conductivity of the carrier gas (H.sub.2) containing UF.sub.6.
The U.S. Pat. No. 2,329,840 describes a measuring device for detecting CO.sub.2 and H.sub.2, in which four chambers are provided as bores in a metal block. The gas to be analyzed is fed into two of these chambers; the two other ones contain a reference gas. The chambers are heated by means of electrical wires. The thermal conductivity of the gas to be analyzed depends upon the concentration of, e.g., CO.sub.2 therein.
This property is used by determining the heat flow from a heated wire into the surrounding gas. On the basis of the four chambers, one provides four such wires and connects them in a bridge circuit; and differences in the thermal conductivity of the gases surrounding the wires imbalances in the bridge. Upon proper adjustment, the bridge voltage is, in fact, a representation of the concentration to be measured.
It could be deemed obvious to use such a cell system for determining UF.sub.6 in H.sub.2. Generally speaking, there is a need in the field of atomic engineering, particularly in the field of Uranium enrichment, to measure the concentration of UF.sub.6 in a carrier gas. It was found, however, that the known devices of the type described do not perform satisfactorily. The reason for this is that even a minor and spurious presence of water vapor renders UF.sub.6 very aggressive and the cells corrode reapidly.
Aside from the foregoing, an analysis of the problem at hand has revealed that the measuring equipment needed for detecting UF.sub.6 in hydrogen must possess these properties: (1) It must be highly insensitive to pressure variations in the operating range which extends from approximately 200 millibars to approximately 300 millibars. (2) The passage ways for the gas must be tightly sealed; in terms of leakage rate values, they must not exceed 10.sup.-8 mbar l/s. (3) The device must operate rather independantly from particulars of the gas flow. (4) Reaction response to changes in concentration must be high (for example, the time of response to a 90% concentration must be below one minute). (5) The device must operate with little dependency upon ambient temperature. (6) The measurement must yield stable outputs.